This invention relates to the construction and operation of high temperature devices. More particularly, it relates to the insulation of furnaces where heat flow through the walls must be minimized.
Currently, most high temperature furnaces are insulated with either cast refractory material or with refractory brick consisting chiefly of alumina, silicon carbide, silicon oxide, or similar materials. While these and related materials are capable of withstanding high temperatures, they also have relatively high thermal conductivity. For example, at approximately 2000.degree. F., the thermal conductivity of alumina brick is approximately 3 BTU/ft.degree.F. hr. Brick of the same material in a foamed configuration has a thermal conductivity of approximately 0.25 BTU/ft.degree.F. hr. (Volume 1 of Industrial Furnace, W. Trinks and M. H. Mawhinney, John Wiley (5th Edition 1961). This material has limited load bearing capability at high temperature. The thermal conductivity for air at the same temperature is approximately 0.04 BTU/ft.degree.F. hr.
A principal use of refractory material in furances is to reduce the temperature to a point where effective insulating materials which have much lower thermal conductivities can be used. Such insulators typically cannot be used at a temperature in excess of about 1000.degree. F. Furnace enclosures are thus frequently built of two layers: a refractory inner layer capable of withstanding very high temperature which in use holds a significant heat gradient, and an outer layer which is a good insulator.
The materials currently used for insulating high temperature devices also store a significant quantity of heat. For example, some industrial steel reheating furnaces may take as much as 300 hours to heat to operating temperature. In furnaces for the manufacture of solid state equipment, maintenance of a constant temperature with only a one degree variance or less is critical. Accordingly, much fuel is wasted to keep the high temperature devices near operating temperature even when the devices are not being used. Because of these operating constraints, many industrial furnaces are never de-energized even though they may be used in production forty hours per week or less. Repairs to large furnaces or their liners are also put off until the damage is so extensive that the cost of losing up to 600 hours of production is outweighed by the energy loss caused by the damage.